The present invention relates to a sounding method and a wireless communication system, and more particularly, to a sounding method and a wireless communication system capable of classifying stations into groups and performing sounding in a group by group fashion.
As a demand for wireless service increases, a very high throughput (VHT) wireless communication system is required for providing more wireless service and faster data transmission rate in the future. In a VHT system, such as IEEE 802.11ac, a multi-user multiple input multiple output (MU-MIMO)/beamforming technology is adopted to enhance a system transmission rate of the VHT system. The MU-MIMO/beamforming technology is able to provide multiple data streams to multiple stations (or users) at the same time. Specifically, the MU-MIMO/beamforming technology allows a base station equipped with multiple antennas to form multiple spatial beams toward multiple stations to provide the multiple data streams to the multiple stations, respectively, such that the system performance, e.g., transmission rate, of the VHT system is enhanced.
Performance of the MU-MIMO/beamforming technology relies on accurate channel state information (CSI) obtained at the base station. The base station needs to perform sounding with the stations, which is a procedure involving the stations sending their CSI back to the base state, before performing data transmission via beamforming. For example, please refer to FIG. 1, which is a schematic diagram of a sounding procedure 10 with respect to time t in the prior art. As shown in FIG. 1, a base station broadcasts a null data packet (NDP) announcement and an NDP to all stations to initial the sounding procedure. Then, the base station would designate one station at a time, by sending a beamforming report poll or the NDP announcement, and the designated station would send back the CSI through a compressed beamforming packet to the base station. After the base station receives the CSIs from all the stations, the base station produces a steering pre-coder matrix to steer the multiple spatial beams to multiple stations and performs data transmission. In other words, each user needs to wait until all stations finish sending their CSIs to the base station, and then receives data transmitted from the base station. A length of a sounding overhead, which is a time period from the base station sending the NDP announcement to the base station starting to transmit data, is so long that system performances, such as throughput, delay jitter, quality of service (QoS) requirement, etc., are degraded. In addition, the length of the sounding overhead is even longer when a number of stations increases. The sounding protocol in the prior art lacks a mechanism of limiting the number of stations to prevent the sounding overhead from being increased. Therefore, the prior art needs to be improved.